According to the video shown at Activision’s E3 booth on a 110″ screen, the mass event in Ghosts is caused by hacking of the “Odin Space Station.”
USA loses control over this space station, and someone hacks into it. US government has only minutes to warn and evacuate cities before the station shoots “rods” at the cities. The first target is New Orleans.
Here’s another image showing off other potential targets: